Heart Block

The common causes of heart block are coronary artery disease, cardiomyopathy and, particularly in elderly people, fibrosis of conducting tissue. Block in either the atrioventricular (AV) node or the His bundle results in AV block, whereas block lower in the conduction system produces right or left bundle block.

Atrioventricular block

• There are three forms:
• First-degree AV block: This is the result of delayed atrioventricular conduction and reflected by a prolonged PR interval (>0.22s) on the ECG. No change in heart rate occurs and treatment is unnecessary
• Second-degree AV block: This occurs when some atrial impulses fail to reach the ventricles
• Mobitz type 1 block (Wenckebach block phenomenon) is progressive PR interval prolongation until a P wave fails to conduct, i.e. absent QRS after P wave. The PR interval then returns to normal and the cycle repeats itself
• Mobitz type 2 block occurs when a dropped QRS complex is not preceded by progressive PR prolongation. Usually the QRS complex is wide
• 2:1 or 3:1 advanced block occurs when every second or third P wave conducts to the ventricles
• Progression from second-degree AV block to complete heart block occurs more frequently following acute anterior myocardial infarction and in Mobitz type II block, and treatment is with a cardiac pacemaker. Patients with Wenckebach AV block or those with second-degree block following acute inferior infarction are usually monitored.
• Complete heart block occurs when all atrial activity fails to conduct to the ventricles. There is no association between atrial and ventricular activity; P waves and QRS complexes occur independently of one another on the ECG. Ventricular contractions are maintained by a spontaneous escape rhythm originating below the site of the block in the:
• His bundle - which gives rise to a narrow complex QRS (<0.12s) at a rate of 50-60 bpm and is relatively reliable. Recent onset block due to transient causes, e.g. ischaemia, may respond to intravenous atropine without need for pacing. Chronic narrow-complex AV block usually requires permanent pacing.
• His-Purkinje system (i.e. distally) gives rise to a broad QRS complex (>0.12s), is slow (<40 bpm), unreliable and often associated with dizziness and blackouts (Stokes-Adams attacks). Permanent pacemaker insertion is indicated.

Bundle branch block 

• Complete block of a bundle branch associated with a wide QRS complex (more than or equals to 0.12s) with an abnormal pattern and is usually asymptomatic. The shape of the QRS depends on whether the right or the left bundle is blocked:
• Right bundle branch block (RBBB) - the right bundle branch no longer conducts an impulse and the two ventricles do not receive an impulse simultaneously. There is sequential spread of an impulse (i.e. first the left ventricle then the right) resulting in a secondary R wave (RSR')  in V1 and a slurred S wave in V5 and V6. RBBB occurs in normal healthy individuals, PE, RVH, IHD, and congenital heart disease, e.g. atrial and ventricular septal defect and Fallot's tetralogy
• Left bundle branch block (LBBB) - the opposite occurs with an RSR' pattern in the left ventricular leads (I, AVL, V4-6) and deep slurred S waves in V1 and V2. LBBB indicates underlying cardiac pathology and occurs in IHD, LVH, and aortic disease and following cardiac surgery